Kayode Odejayi should have done a lot better than blaze high and wide after being released by an excellent Jon Macken pass from the left, but the same can be said for Watford's stand-in left-sided midfielder Jordan Stewart.
He saw his opportunity to latch onto Lloyd Doyley's cross from the right and drifted in unmarked round the back of the Tykes defence, only to head badly wide of Luke Steele's near post.
However, the manner in which the Hornets fell behind would have left manager Aidy Boothroyd fuming.
Instead of closing down the loose ball after a Diego Leon was shot blocked, they simply stood and watched as Leon's lovely flick found Macken on the right side of the area and his slide-rule pass across the six-yard box presented Odejayi with the simplest of finishes.
And within ten minutes of the restart it was game over. First, Steve Foster was left completely unmarked at the far post to head home Leon's corner, and then the midfielder released Odejayi with a majestic pass that the hero of the Chelsea victory finished in style.
Quite where this defeat leaves Watford's promotion prospects is anyone's guess, but a trip to The Hawthorns to face a free-scoring West Bromwich Albion would not be the opposition of choice next up, particularly as Boothroyd's men have now conceded a fifth of their goals against the teams in the bottom three prior to kick-off.